Water desalination

ABSTRACT

A desalination plant is disclosed which includes a pump for pumping water at a pressure of between 50 and 65 Bar to a generally cylindrical filter element which includes a plurality of reverse osmosis membranes defining salt passages. Immediately upstream of the filter element there is a disc with a plurality of holes in it. The disc forms an obstruction which causes a pressure drop between the upstream side thereof and the downstream side. It also divides the water flow into a series of separate streams which impinge on the end of the filter element and flow into the salt passages. The water downstream of the obstruction is not only at a lower pressure than the water upstream of the obstruction but also is flowing turbulently. The disc and filter element are in a cylindrical casing. The brine which emerges from the filter element, and which is still at a substantial pressure, can be fed through a device such as a Pelton wheel to recover some of the residual energy therefrom.

This application is a continuation of Ser. No. 08/817,235, filed on Apr.10, 1997, now U.S. Pat. No. 6,139,750, which is a 371 of PCT/GB96/03060filed Dec. 13, 1996.

FIELD OF THE INVENTION

THIS INVENTION relates to water desalination, that is, to the removal ofdissolved solids from sea water and brackish water.

BACKGROUND TO THE INVENTION

Discussions on the world's shortage of drinking and irrigation water arecommonplace. In some parts of the world whole cities are going to haveto be abandoned because of prolonged drought.

The only inexhaustible supply of water is the sea but desalination ofwater in significant quantities to supply major population centres orlarge scale irrigation projects is costly. Many desalination plantsoperate on the basis of reverse osmosis. In this type of plant the waterto be desalinated is forced through a semi-permeable membrane so thatthe dissolved solids are removed by the membrane. Other plants operateon the basis of evaporation.

A major problem with both the methods described is that the waterobtained is, in the case of the evaporation method, pure distilledwater, and in the reverse osmosis method is of the same degree of purityas distilled water. It has virtually all the minerals that weredissolved therein removed. Water without any calcium or magnesium in itis aggressive towards metal pipes and other metal objects with which itcomes into contact. Hence these minerals must be added to the recoveredwater. Furthermore distilled water is tasteless and, being devoid ofessential minerals, cannot be used for human consumption over aprolonged period. Hence, for drinking purposes, it is necessary to add arange of minerals to convert the water from “flat” distilled water toacceptable drinking water. In both methods described the essentialminerals which were present in the sea water are in the brine which is aby-product of the process. A significant cost in producing water fromeither type of plant is thus the cost of the minerals which must bereintroduced into the water and the equipment needed for this purpose.

In an evaporation plant the power needed to evaporate the sea water isalso significant when costs per megalitre of recovered water arecalculated.

Reverse osmosis membranes are of composite construction and oneextensively used form comprises two films of a complex polymeric resinwhich together define a salt passage. In the passage there is an elementfor inducing turbulence in the flow. The element is usually a weldedmesh of plastics material filaments. A number of these membranes arewound in a complex manner onto a central tube. Water which passesthrough said films enters spaces between adjacent membranes and flows tothe central tube. The tube has apertures in the wall thereof to permitthe recovered water to enter the tube. The brine, that is, the residueof the sea water and the bulk of the dissolved solids flows out of themultitude of salt passages to waste or to a salt recovery plant.

It is accepted by those working in this art that on each side of eachsalt passage, and immediately adjacent each film, there is aconcentration polarization layer. These layers, which are ofmulti-molecular thickness, contain a higher concentration of dissolvedsolids than the bulk flow in the part of the salt passage mid-waybetween the films. The turbulence inducing element is intended to reducethe thickness of the concentration polarization layer and hence enhancethe ability of the membrane to allow water to permeate through it.Typically a state of the art reverse osmosis membrane will achieve a99.3% dissolved solids rejection rate. The dissolved solids that passthrough the membrane largely consist of common salt as its molecules aresmaller than the molecules of most other minerals. A percentage of 0.7%represents 400-500 parts per million of dissolved solids in therecovered water, depending on the initial salinity of the sea water, andis below the threshold at which the dissolved solids impart taste to thewater.

Fouling of reverse osmosis membranes is a major problem and measureswhich increase the cost of water production have to be taken to inhibitfouling and to remove it when it does occur. Fouling can result frommineral deposition in the membrane or from organic growth. By way ofexample, before the sea water reaches the membrane it is treated with aninhibitor such as sodium hexametaphosphate (known commonly as “shrimp”).This limits calcium and magnesium precipitation on the membrane in theform of calcium and magnesium carbonates but adds another factor toproduction costs.

Membrane manufacturers recommend a relatively low flux rate (rate ofwater flow through a membrane in litres per hour per square metre ofmembrane) to avoid rapid fouling. Back-washing of a membrane, that is,causing water to flow in the reverse direction through the saltpassages, is a standard procedure for removing fouling. If a membrane isheavily fouled it must be removed from the recovery plant and subjectedto a variety of treatments for the purpose of removing the fouling. Inextreme cases the fouling cannot be removed and the membrane has to bediscarded.

As a result of all these factors water produced from a reverse osmosisplant is more costly than water obtained by purifying water from astorage dam or river. Hence, despite the world's shortage of water, onlya small percentage of the world's water is produced using reverseosmosis plants to desalinate sea water.

OBJECT OF THE INVENTION

The main objects of the present invention are to improve the efficiencyof the reverse osmosis process, significantly to reduce the cost ofwater produced by the reverse osmosis process, to inhibit fouling ofreverse osmosis membranes and to produce water with desirable mineralstherein without the necessity for dosing.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the present invention there is provided areverse osmosis desalination plant which comprises a filter elementconsisting of reverse osmosis membranes defining salt passages, a pumpfor pumping water to be desalinated to said filter element, and anobstruction in the water flow path between said pump and said filterelement for introducing turbulence into the flowing water and causing apressure drop across the obstruction whereby the water downstream of theobstruction as it enters said salt passages of the filter element is ata lower pressure than the water upstream of the obstruction and its flowis more turbulent than it was upstream of the obstruction.

The obstruction is preferably in the form of a plate with a plurality ofholes in it whereby the flowing water is obstructed and divided up intoa number of conical, diverging turbulent water streams each of which isat a lower pressure than the pressure of the water upstream of theplate. The holes in the plate can be of different sizes or can all be ofthe same size as one another. In a preferred form the plate is in theform of a circular disc and the holes are in a spiral array about thecentre of the disc. In another form the holes are in a circular arrayand in yet another form the holes lie along lines radiating out from thedisc centre.

If desired a series of flow restricting valves can be provided forvarying the flow areas of the holes in the plate which create theindividual water streams.

According to a further aspect of the present invention there is provideda method of desalinating water which comprises pumping water to bedesalinated to a filter element consisting of reverse osmosis membranesdefining salt passages, causing a pressure drop in the water flowing tothe filter element and simultaneously introducing turbulence into thewater flow, and feeding the turbulent water at the lower pressure intothe salt passages of the filter element.

In the preferred embodiment the water is divided into a plurality ofturbulent conically shaped, diverging water streams by said obstructionwhich drops the pressure and introduces the turbulence, each turbulentstream impinging on the filter element.

It has been found that inlet pressures in the range 50 to 65 Bar and apressure drop of between 1.5 and 2.0 Bar provide the best results.

The plant and method according to the present invention recover waterwhich has acceptable levels of dissolved solids, that is, minerals init. No dosing of the recovered water is required as it has thereinsufficient dissolved solids to give it an acceptable taste. Becausemagnesium and calcium are present in the recovered water it is notaggressive towards metal pipes and fittings and no dosing with theseminerals is required.

It is believed that by introducing water which is flowing in a turbulentmanner into the salt passages of the membranes, the concentrationpolarization layer is reduced in thickness. This enables the flux rateto be increased without unduly increasing fouling. A further effect isto allow through the membrane minerals in addition to common salt whilstnot increasing the quantity of common salt in the recovered water to anunacceptable level. Experimental work has shown that by varying thepressure drop and the turbulence, for example by varying the hole sizesin the plate when this forms the obstruction, different dissolved solidscan be caused to pass through the membranes in controllable quantities.Hence by trial and experiment i.e. by varying the pressure drop andturbulence, water having dissolved solids in predetermined quantitiescan be recovered.

A further advantage is that experimental work has shown that fouling ofthe membrane is significantly reduced when turbulent water is fed to it.

The brine which emerges from a conventional reverse osmosis plant isheavier than sea water and hence sinks if fed back into the sea.However, the brine emerging from a desalination plant in accordance withthe present invention, when fed back into the sea, initially rises inthe form of a plume instead of sinking. The brine has been found to beaerated, and the aerating agent has been found to be oxygen.Furthermore, there are oxygen bubbles in the recovered water.

Tests show that there is more oxygen in the recovered water and in thebrine than there should be based on the amount of oxygen dissolved insea water. The oxygen bubbles are small because, even downstream of theobstruction, there is substantial pressure, for example, 45 to 50 Bar.The small bubbles in the turbulent water are believed to play a part inreducing the thickness of the concentration polarization layers. Thebubbles also seem to play a part in preventing fouling of the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how thesame may be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings in which:

FIGS. 1A and 1B together constitute an axial section through adesalination unit which forms part of a desalination plant;

FIG. 2 is a section, taken in the same plane as that of FIGS. 1A and 1B,and showing one end part of the unit to a larger scale;

FIG. 3 is an elevation of a disc;

FIG. 4 is a section, taken in the same plane as FIG. 2 and to the samescale, and showing a modification of the unit of FIGS. 1A and 1B;

FIGS. 5A and 5B illustrate further discs;

FIG. 6 is a diagrammatic cross section through a hand operated waterdesalination plant;

FIG. 7 diagrammatically illustrates a motor driven desalination plant;

FIG. 8 diagrammatically illustrates a further desalination plant;

FIG. 9 is a diagrammatic representation of a submersible desalinationplant;

FIG. 10 is a diagram illustrating the layout of a water desalinationplant;

FIG. 11 illustrates a submersible desalination plant;

FIGS. 12A and 12B together illustrate a desalination plant which iswithin a single outer casing;

FIG. 13 illustrates a floating desalination plant; and

FIG. 14 illustrates a tank and an associated piping system.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring firstly to FIGS. 1A and 1B, the desalination unit illustratedis generally designated 10 and comprises a cylindrical casing 12 withend caps 14 and 16 secured in opposite ends thereof. An inlet pipe 18for water with solids dissolved in it passes through the end cap 14 andfeeds water into a chamber 20. The pipe 18 is connected to the pressureside of a pump (not shown in FIG. 1A) capable of delivering water at,say, 50 to 65 Bar. A brine outlet pipe 22 leads from a chamber 24through the end cap 16. Lip seals 26 and 28 encircle the end caps 14 and16 and seal between the caps 14 and 16 and the casing 12.

Reference numeral 30 generally designates a reverse osmosis filterelement which fits snugly into the casing 12. The element 30 comprises acore structure 32 including a central tube 34 which forms the recoveredwater outlet of the filter element 30. The tube 34, which has aplurality of holes 36 therein, passes at one end thereof through the endcap 16. The other end of the tube 34 is supported in a blind socket 38(see also FIG. 2) provided therefor in a supporting plate which is inthe form of a disc 40. The disc 40 and cap 14 form the bounding walls ofthe chamber 20. A lip seal 42 encircles the disc 40 between the disc 40and the casing 12. There is a gap (see FIG. 2) between the disc 40 andthe filter element 30.

The filter element 30 comprises, in addition to the core structure 32, asemi-permeable membrane which is wound onto the core structure 32. Thewound membrane fills the entire space between the core structure 32 andthe internal face of the casing 12 and, apart from the gap between itand the disc 40, fills the space between the disc 40 and the chamber 24.

A commercially available form of filter element which is suitable foruse in the present invention is that manufactured and sold by FilmtecCorporation which is a wholly owned subsidiary of Dow Chemical Company.The product carries the designation FT30. U.S. Pat. No. 4,277,344describes in detail a membrane which operates on the reverse osmosisprinciple. The winding of the membrane of the filter element 30 iscomplex. It is initially formed into a series of flattened pockets whichare then wound onto the core structure 32 in an overlappingrelationship.

The disc 40 (see FIG. 3) has a series of eight holes 44.1, 44.2 etctherein. The holes vary in size and, in the illustrated embodiment holesof 8,805 mm, 9,185 mm, 8,077 mm, 7,772 mm, 7,675 mm, 7,351 mm, 7,094 mmand 7,881 mm are used. The diameter of the disc 40 is about 20 cm whichis also the inside diameter of the casing 12 and the outside diameter ofthe filter element 30.

Behind the disc 40, and between it and the wound membrane, is a spider46 (shown in outline in FIG. 3) comprising a central hub, an outer ringand a plurality of spokes extending between the hub and the ring. Thespider 46 is part of the filter element as available from Filmtec anddefines a series of wedge shaped openings. Each hole 44.1, 44.2 is inregister with one of those openings so that each water stream impingeson the filter element.

When water under pressure flows through a restricted hole underpressure, the stream of water emerging from the hole flares out intoconical form and then at a distance from the hole breaks up intodroplets. The conical part of the stream of water between the hole andthe point where the stream breaks up is itself turbulent having eddycurrents and vortices in it. The filter element 30 is positioned so thatthe streams of water emerging from the holes 44.1 etc impinge on thefilter element and flow into the salt passages before they break up intosprays of droplets. Break up is inhibited in the unit illustratedbecause, immediately water starts to flow, the gap between the disc 40and element 30 fills with water under pressure.

Applicant has found that water fed at the specific pressures describedinto the filter element 30 does not have 99.3% of the dissolved solidsremoved instead a lower percentage is removed. With an inlet pressure of50 bar and a disc 40 as described above the system desalinates sea waterto a potable water which meets the standard set in South African Bureauof Standards Specification 241-1984.

Pressures in the range of about 48.5 Bar to 49.5 Bar are obtaineddownstream of the holes 44.1, 44.2 etc with a pressure in the chamber 20of about 50 Bar. Applicant has also detected a very slight temperatureincrease across the disc 40 and assumes that this results from theintroduction of turbulence into the flow.

The structure of FIG. 4 differs from that of FIGS. 1A, 1B, 2 and 3 inthat the different pressures on the downstream side of the disc 40 areachieved by the settings of a circular array of water flow controlvalves 48. The valves 48 include shutters or diaphragms for varyingtheir effective flow areas, and together constitute the obstructionwhich introduces turbulence and causes the pressure drop. Each valve 48has a control cable 50 leading to it and each valve 48 is in a pipe 52.The pipes 52 are of the same diameter as one another and pass throughthe disc 40. The valves 48 are electrically operated and the degree bywhich they are open can be controlled from a programmable controller.The setting of each valve 48 determines the pressure at the outlet ofthe respective pipe 52. Varying the pressure by means of the controllerenables the dissolved solids in the recovered water to be varied asdesired. Whilst the valves have been shown to the rear of the disc 40they would, in a usable construction, be within the disc and adjacentthe outlets from the holes in the disc 40.

The disc 40 of FIG. 3 has the holes therein arranged in a circulararray. In FIG. 5A the holes are in a spiral array concentric with thedisc. The spiral turns in the same direction as the filter element 30 iswound. In FIG. 5B the holes are arranged on a number of radial lines.The holes in FIGS. 5A and 5B are smaller than those shown in FIG. 3 andmore numerous.

Referring now to FIG. 6, the hand operated water desalination plant 54illustrated comprises a cylindrical casing 56 which has therein acommercially available filter element 58 such as that described aboveand designated 30 in FIGS. 1A and 1B. A seal 60 encircles the filterelement 58 to prevent water leaking between the casing 56 and the filterelement 58. Adjacent one end face of the filter element 58 there is adisc 62. Between the disc 62 and the casing 56 there is a seal 64.Movement of the disc 62 to the left is prevented by a retaining ring 66.

The holes in the disc 62 are not shown. There is a gap between the disc62 and the filter element 58.

Adjacent the other end of the filter element 58 there is an end cap 68which has a tapped central bore 70 through it and a subsidiary bore 72which is to one side of the bore 70.

The filter element 58 is shown as having a central tube 74 protruding inopposite directions from the wound membrane thereof. One end of the tube74 is seated in a blind recess 76 in the disc 62 and the other end ofthe tube 74 enters the bore 70 of the end cap 68. The bore 72 is incommunication with a chamber designated 78 which is between the end cap68 and the adjacent end of the filter element 58.

The filter element 58, disc 62 and end cap 68 are as illustrated inFIGS. 1A and 1B and hence these components form a desalination unit 10.

To the left of the disc 62 the casing 56 forms a barrel for a piston 80.The piston 80 includes a rod 82 and this emerges from the casing 56through a sealing structure designated 84. A spider 86 holds the sealingstructure 84 in place.

Two lip seals 88 and 90 and an O-ring 92 encircle the piston 80.

An operating handle 94 is connected to the rod 82 by means of a slidingcoupling (not shown). A link 96 pivotally connects the handle 94 to anend plate 98 which is itself secured to the flange 100 of the casing 56.By oscillating the handle 94, the piston 80 can be reciprocated inforward and return strokes in its barrel.

The bore 72 is connected by a pipe 102 to a chamber 104 which encirclesthe rod 82 and sealing structure 84.

A one way valve 106 allows water to enter a chamber 108 which is betweenthe disc 62 and the piston 80. The valve 106 is mounted in an opening inthe walling of the casing 56 and a pressure relief port 110 is alsoprovided in the walling of the casing 56.

An outlet pipe (not shown) is screwed into the tapped bore 70 andrecovered potable water flows from the tube 74 into this outlet pipe.

In use of the desalination plant illustrated in FIG. 6, the casing 56 isfixed with the valve 106 immersed in the salt water or brackish waterthat is to be desalinated. The upper end of the handle 94 is pushed orpulled to the position illustrated which moves the piston 80 in itsreturn stroke. As the piston moves to the left the valve 106 opens andbrackish or salt water is drawn into the chamber 108. When the handle 94is pushed to the left the piston 80 commences its working stroke andmoves towards the disc 62. The valve 106 closes immediately the pressurerises in the chamber 108. The water in the chamber 108 is forced throughthe holes in the disc 62, through the filter element 58 and out of thefilter element as potable water via the tube 74 or as brine through thebore 72 and pipe 102. The piston 80 continues to move to the right untilthe lip seal 90 has passed the valve 106.

After a few strokes of the handle 94 pressure begins to build-up in thepipe 102 and hence in the chamber 104. The forward stroke of the piston80 is eventually assisted by the pressure existing in the pipe 102 andchamber 104. As the piston 80 reaches the forward end of its stroke, thelip seal 88 moves past the pressure relief port 110 and the pressure inthe chamber 104 drops. Thus the return stroke of the piston 80 is notresisted by any pressure in the chamber 104.

The pressure required to force water through the filter element 58 andseparate it into a stream of potable water and a stream of brine is inthe order of fifteen to twenty five Bar (for brackish water) and fiftyto sixty Bar (for sea water). The pressure required varies with theamount of dissolved solids in the water. The pressure loss in the filterelement 58 is relatively small and the pressure of the brine in the pipe102 can be 75% to 85% of the pressure which exists where the waterenters the filter element 58. This excess pressure, which wouldotherwise be lost, is used as described to assist in operation of thepump.

Turning now to FIG. 7, the desalination plant illustrated comprises acasing 112 which is arranged vertically. The ends of the casing areclosed by end caps 114 and 116 and there are sealing rings (not shown)between the end caps 114 and 116 and the casing 112. Immediately belowthe cap 114 there is a chamber 118 and a disc 120. Below the disc 120there is a filter element 122. There is a gap 124 between the disc 120and the filter element 122.

The filter element 122 has a central tube 126. The upper end of the tube126 is located by the disc 120 and the lower end of the tube 126 islocated by the end cap 116. An inlet pipe 128 leads into the chamber118. A brine outlet pipe 130 leads through the end cap 116 and a potablewater outlet pipe 132 passes through the end cap 114 and connects to theupper end of the tube 126. The disc 120 is, for example, of theconfiguration shown in FIG. 3, FIG. 5A or FIG. 5B. The componentsdescribed constitute a desalination unit 10.

A vertically arranged pump 134 of the Grunfos type has its suction inlet136 connected by way of a filter 138 to a pond or other source of waterto be desalinated. The pipe 128 is connected to the pressure outlet ofthe pump 134, there being a control valve 140 in the pipe 128.

The pipe 130 is connected via a T-piece 142 and a control valve 144 to aPelton wheel 146. The other limb of the T-piece 142 is connected via acontrol valve 148 to a waste outlet 150 from which brine is dischargedto waste. The outlet side of the Pelton wheel 146 also discharges towaste.

The motor of the pump 134 is designated 152. Its electrical supply cancomprise, as alternatives, a direct connection to a 220 volt main or aconnection to a solar panel 154, a battery 156 and an inverter 158. Acontrol 160 for enabling the rate at which the motor 152 is driven to bevaried is provided in the supply circuit.

The Pelton wheel's central shaft is connected to the drive shaft of themotor 152. As explained above with reference to FIG. 6, there is apressure drop within the filter element 122 but the brine emerging fromthe filter element 122 is still at substantial pressure. By feeding someor all of the brine under pressure through the Pelton wheel, the powerrequirements of the motor 152 can be reduced by using some of thepressure energy that would otherwise be lost.

In FIG. 8 there is illustrated a plant which is similar to that of FIG.7 and like parts have been designated with like reference numerals. Inthis form the water to be desalinated enters at the bottom of the casing112 instead of the top and the pump and motor (designated 162 and 164respectively) are not an integral unit. They are, instead, mountedside-by-side by means of their base plates 166 and 168. The pressureinlet to the casing designated 112 is by way of the pipe 128. Thedesalinated water emerges through the pipe 132 and the brine emergesthrough the pipe 130.

The Pelton wheel 146 assists in driving the pump 162.

The desalination plant shown in FIG. 9 comprises a vertical main casing170 which is placed at the bottom of a borehole having brackish watertherein or at the bottom of a pool containing sea water. A pump is shownat 172 and the motor which drives the pump is shown at 174. The pressureside of the pump is connected to a chamber 176, the upper end of thechamber 176 being constituted by a disc 178. Above the disc 178 is afilter element 180.

Above the filter element 180 there is an end cap 182 which bounds achamber between itself and the filter element 180. Brine emerging fromthe filter element 180 enters this chamber and recovered water emergesfrom the filter element 180 through a pipe 184.

A Pelton wheel 186 is mounted on the casing 170 above the end cap 182.

The chamber between the end cap 182 and the filter element 180 isconnected by a pipe 188 to the Pelton wheel. It will be understood thatthere is considerable pressure in the chamber. The brine entering thischamber under pressure from the filter element 180 is fed through thepipe 188 and the Pelton wheel 186 to a discharge pipe designated 190.The Pelton wheel 186 drives a pump (not shown). The pump is axiallyaligned with the Pelton wheel 186 and the pipe 184 is connected to thepump. The purpose of the pump driven by the Pelton wheel is to lift therecovered water up to ground level via a hollow column 192 (if thecasing 170 is in a borehole) or up to the surface of the pool (if thecasing 170 is immersed in a salt water pool).

The motor 174 is powered from an array of solar panels 194 which areused to charge batteries 196. A 220 volt supply is shown at 198. This isconnected to a step down transformer and rectifier 200. It is alsoconnected to a control unit 202 through which power is fed to the motor174. The panels 194 and rectifier 200 serve to charge the batteries 196.The output from the batteries 196 is fed through an inverter 204 whichconverts the 12 volt d.c output of the batteries to 220 volt AC. Achange over switch 206 enables power to be taken from the inverter 204or from the power supply 198 depending on how much power is available inthe batteries. The control unit 202 steps-up the 220 volt input voltageto a 380 output voltage for feeding the motor 174.

An advantage of the plant of FIG. 9 is that only the recovered water islifted to the surface.

The plant shown in FIG. 10 comprises a casing 208 with a filter element210 therein. The inlet for water to be desalinated is at 212 and theoutlet for brine is shown at 214. The outlet for recovered water isshown at 216. The means for causing the pressure drop upstream of thefilter element 210 and for creating the streams of water which impingeon the filter element 210 is shown as being of the form illustrated inFIG. 4.

The supply of water to be desalinated is shown at 218 and can be a seawater pool or a source of brackish water. A feed pump is shown at 220,this extracting water from the supply 218 and feeding it through a sandfilter 222 and a disc filter 224. A high pressure pump is shown at 226,the suction side of this being connected to the filter 224 and thepressure side to the inlet 212.

The outlet 216 is connected to a vessel 228 in which the recovered wateris subjected to ultraviolet light (UV). Exposure of the water to UV is astandard procedure in water purification. The outlet from the vessel 228leads to a storage tank 230.

In the event that the plant is not to be run for a period of time, forexample, because there is sufficient recovered water in storage, thereis a risk of bacteria and algae growing in the element 210. This canonly be avoided by the continual circulation of water through theelement 210. For this purpose the tank 230 can be connected via a pump232 and a valve 234 to the inlet 212. A valve 236 is closed when thevalve 234 opens. Using this circuit it is possible continually tocirculate recovered water through the element 210 thereby to ensure thatbacterial growth is inhibited. As the pressure which the pump 232produces is relatively low, there is a “washing” action but the pressureis insufficient to force water through the membranes and thence to thetank 230. The water used for washing purposes is discharged to waste.

The brine outlet 214 is connected to a Pelton wheel 238 so thatadvantage can be taken of the residual pressure downstream of the filterelement 210. The Pelton wheel can be used to pump recovered water or togenerate electricity or to assist in driving the rotor of either of thepumps 220 or 226.

It is possible to incorporate flow switches 240 which detect when flowis occurring in the pipe in which they are mounted, and flow meters 242which detect the rate of flow. The pH and the conductivity of therecovered water can also be measured (at 244 and 246). All theinformation derived is fed to a master control 248 which exercisesoverall control of the system. Further valves for enabling the pipesinto which they are fitted to be closed are shown at 250, 252, 254, 256,258, 260, 262 and 264.

To backwash the disc filter 224, the valves 234 and 250 are closed andthe valves 236 and 262 opened. Water is thus withdrawn from the tank 230by the pump 232, fed through the open valve 236, forced through thefilter 224 in the reverse direction and discharged to waste through theopen valve 262.

A level detector 266 in the tank 230 can be used to determine when thetank has been filled. The resultant signal can be used to shut-offwithdrawal of water from the supply 218 and initiate recycling throughthe pump 232 and valve 234 to prevent bacterial growth.

The torque of the Pelton wheel 268 can be controlled by incorporating atorque detector 270. If the torque increases above a predeterminedlevel, the valve 256 is opened so that some of the brine by-passes thePelton wheel 268 and flows directly to waste through the valve 256.

The settings of the valves which control water flow to the filterelement 210 can be controlled using a keypad 272 of the type used withP.C.'s.

The plant shown in FIG. 11 comprises a vertically positioneddesalination unit 10 as shown in FIG. 1 standing vertically in a pond274. Like parts have been designated with like references. The inlet forwater to be desalinated is shown at 18, the outlet for desalinated wateris shown as being connected to pipe 34, and the brine outlet is shown at22.

A pump is shown at 276 in FIG. 11. The pump 276 is a verticallyoperating ram pump having its inlet at the upper end and its outlet atthe lower end. An outlet pipe is designated 278 and there is anauxiliary pump 280 in the outlet 278. The motor of the pump 280 isconnected to a solar panel 282. The function of the pump 280 is toinitiate flow through the ram pump 276. It does this by sucking waterthrough the ram pump 276 and discharging it through an outlet pipe 284.

The pump 276 includes flow control valves 286 and 288, one being at theupper end of the pump and the other being at the lower end of the pump.When the pump 276 is started, the resultant downward flow through thepump 276 sucks the valve 286 to the open position and forces the valve288 to the closed position. As the valve 288 closes a shock wave istransmitted through the pump 276. The shock wave forces water under highpressure through a one way valve 290 into the inlet 18 of the casing 12.There is a further one way valve 292 in the inlet 18.

A diaphragm 294 is connected to the valve 290. As the valve 290 opensthe diaphragm is pushed through a dead centre position. Once thepressure shock has dissipated, the diaphragm 294 is effective tore-close the valve 290.

The valves 286, 288 are connected by a rod 296 and thus move in unison.Once flow through the ram pump has been initiated, the pump 280 can beswitched off and left in an open condition so that flow can take placethrough it. The head of water in the pond (bounded by a side wall 298and a bottom wall 300) ensures that the pump 276 continues cycling.

The residual pressure of the brine in the outlet 22 can be used for anyof the purposes described above.

Desirably the wall 298 divides the pond 274 from the sea. When there isa high tide water flows over the top of the wall 298 and fills the pond274. This provides the requisite operating head for the pump 276. As thetide falls, and no more water enters the pond, the level in the pondsteadily drops as water flows away through the ram pump 276 and theoutlet pipe 284.

The submersible desalination plant shown in FIGS. 12A and 12B comprisesa cylindrical casing 302. Within the casing, and at one end thereof,there is an electric motor 304 which drives a pump 306. The pump 304 canbe of any suitable kind eg a piston pump, a swash plate pump etc. Thesalt water inlet to the pump 306 has not been shown but the pump outletis designated 308. The outlet 308 divides into two branches 310 and 312and there are valves 314 and 316 in the two branches 310 and 312. Thebranch 310 leads to the core of a disc filter 318 which is contained ina cavity 320. A disc 322 forms one boundary of the cavity 320 and on theother side of the disc 322 there is a filter element 324. The disc 322can be as described above with reference to FIGS. 1A, 1B, 2 and 3 orFIG. 4, or FIGS. 5A or 5B. The holes in the disc 322 are not shown.

The branch 312 leads directly into the cavity 320 and an outlet 326leads from the core of the filter 318 through the disc 322. The outlet326 has therein a valve (not shown) which is normally closed.

The disc filter 318 can be cleaned by closing the valve 314 and openingboth the valve 316 and the valve in the outlet 326. Thus water flowsinto the cavity 320, from the cavity 320 through the disc filter 318 inthe reverse direction and out through the outlet 326 carrying away anydirt particles that have been trapped in the disc filter 318.

Within the casing 302 the recovered water is subjected to ultra violetlight in a unit 328.

The brine can, as described above, be fed back to the motor and pump sothat its residual pressure can be used to reduce the power requirementsof the motor 304.

The power supply to the motor 304 can be as described above withreference to, for example, FIGS. 7 and 9.

The floating desalination plant shown in FIG. 13 comprises a housing330, an anchor block 332 secured to the sea bed or simply resting on thesea bed and an anchor cable 334 connecting the housing 330 to the anchorblock 332.

A horizontal partition 336 divides a buoyancy space 338 which is abovethe partition 336 from a water intake chamber 340 which is below thepartition 336. Holes 342 in the housing 330 permit sea water to enterthe intake chamber 340.

An electric motor 344 is mounted so that it is largely within thechamber 340 and is thus cooled by the sea water which flows into thechamber 340. Mounted above the motor 344 there is a pump 346 which isdriven by the motor 344. Water is drawn by the pump 346 from the chamber340 through a filter 348.

The pressure port of the pump 346 is connected by piping generallydesignated 350 to three units 10 of the type shown in FIGS. 1A and 1B.Whilst three units 10 are shown within the housing 330 any suitablenumber from one upwards can be used.

Brine emerges from the units 10 through piping designated 352 and isdischarged to waste through an outlet designated 354. Recovered wateremerges through piping generally designated 356 and passes through anultra violet unit 358 to reach an outlet 360. Piping (not shown) runsfrom the outlet 360 to the shore and, in the illustrated embodiment, anelectrical cable (not shown) runs from the shore to supply power to themotor 344.

At the upper end of the housing 330 there is a solar panel 362 which isused to power a light and a radio transmitter generally designated 364.These are intended to warn passing shipping of the hazard constituted bythe floating plant.

To make it unnecessary to provide power to the plant and enable themotor 344 and pump 346 to be omitted, a piston pump can be providedbetween the casing 330 and the anchor block 332. More specifically, arod (not shown) can extend downwardly from the housing 330 and have apiston at the lower end thereof. A cylinder is mounted on the anchorblock 332, the piston being within the cylinder. The piston and cylinderconstitute a pump which can be double acting or single acting.

It will be understood that the housing 330 will rise and fall through adistance which depends on the magnitude of the swells passing it. As thehousing 330 rises it lifts the piston rod and piston with respect to thecylinder which is prevented from lifting by the anchor block. A lowerchamber of the cylinder thus increases in size and can be filled withsea water through a non-return valve. As the housing 330 drops into atrough between two swells, the piston moves down the casing reducing thevolume of said lower chamber. A further one-way valve opens under theinfluence of the increasing pressure in the lower chamber and sea wateris forced from the lower chamber into the piping system 350. If desiredthe piston rod can be hollow and this can form the flow path from thelower chamber to the system 350.

The upper chamber of the cylinder can simply be opened to the sea.However, it is preferred that it also has a one way inlet valve and aone way outlet valve so that water is pumped both when the piston isdropping with respect to the cylinder and when it is lifting withrespect to the cylinder.

Referring finally to FIG. 14, reference numeral 366 designates avertically elongate tank which has a sea water inlet 368 through whichsea water is pumped into the tank. The tank is open at its upper end toprovide an air vent 370. An outlet 372 is connected to the suction inletof a pump which feeds water to the unit shown in FIGS. 1A and 1B. Therecovered water outlet from the unit of FIGS. 1A and 1B is connected toan inlet 372 of the tank 366 so that water with a low concentration ofdissolved solids in it is returned to the tank 366. A further outlet isshown at 374, this enabling the tank to be drained and solids which arein it to be removed. A vertically elongate sight glass is shown at 376.

At start-up of the desalination plant of which it forms a part the tank366 has therein a volume of recovered water which is approximately equalto one third of the volume of water that it will eventually contain. Seawater is pumped in through the inlet 368 and recovered water is fed inthrough the inlet 372. Thereafter water is sucked continuously from thetank 366 through the outlet 372. The sea water which entered through theinlet 368 is diluted before leaving the tank through the outlet 372. Ithas been found that although some of the recovered water is recycled andnot all the recovered water is immediately removed from the plant, thetotal off-take of recovered water increases and lower pressures arerequired to ensure that the unwanted dissolved solids are removed fromthe water.

Experimental work has shown that, whilst recovered water with a lowdissolved solids content can be fed in through the inlet 372, it isdesirable to employ a conventional desalination unit which provideswater which is of the same quality as distilled water as the sourcewhich is connected to the inlet 372.

It has also been found that water produced by the method and apparatusof the present invention can have a small quantity of the brine added toit without this increasing the common salt content to unacceptablelevels. This procedure can be used, for example, where conditions cannotbe established which will leave a sufficient quantity of one mineral inthe water. Supplementing the mineral which is not present in sufficientquantities by adding brine is then a possible method of achieving therequisite mineral balance.

What is claimed is:
 1. A reverse osmosis desalination plant, comprising:a casing; a filter element located within said casing and comprisingreverse osmosis membranes defining salt retention passages; a pump forpumping water to be desalinated to said filter element; and a platelocated within said casing and in the water flow path between said pumpand said filter element, said plate comprising a plurality of holeshaving circular cross-sections, there being holes in the plate which aredifferent diameters to one another thereby to produce individual waterstreams of different areas, the pressure drop across the plate and theturbulence introduced into the water as it flows through the holes beingsuch that gases dissolved in the water come out of solution as bubbleswhich enter said salt retention passages entrained in the flowing water,the water downstream of the plate as it enters said salt retentionpassages of the filter element with said bubbles entrained in it beingat a lower pressure than the water upstream of the plate and its flowbeing in the form of turbulent streams.
 2. The plant according to claim1, wherein the plate comprises a circular disc and the holes arearranged in a series of circles the centers of which are coincident withthe center of the disc.
 3. The plant according to claim 1, wherein theplate comprises a circular disc and the holes are arranged in the formof a spiral generated about the center of the disc.
 4. A reverse osmosisdesalination plant, comprising: a casing; a filter element locatedwithin said casing and consisting of reverse osmosis membranes, saidmembranes being in the form a plurality of leaves attached to and woundaround a permeate pipe and defining a plurality of salt retentionpassages and a plurality of permeate passages each of which passages isof spiral form; a pump for pumping water to be desalinated to saidfilter element; and a plate located within said casing and in the waterflow path between said pump and said filter element, said plate havingtherein a plurality of holes of circular cross section through whichindividual water streams flow, each stream impinging on the end of thewound membrane and flowing into the salt retention passages, thepressure drop across the plate and the turbulence introduced into thewater by the plate being such that gases dissolved in the water come outof solution as bubbles which enter said salt retention passages in thewater flow, the water downstream of the plate as it enters said saltretention passages of the filter element with said bubbles entrained init being at a lower pressure than the water upstream of the plate andits flow being in the form of turbulent streams.
 5. A reverse osmosisdesalination plant, comprising: a casing; a filter element locatedwithin said casing and consisting of reverse osmosis membranes definingsalt retention passages; a pump for pumping water to be desalinated tosaid filter element; and a plate located within said casing and in thewater flow path between said pump and said filter element, said platehaving a plurality of holes of circular cross section therein, therebeing holes in the plate which are different diameters to one anotherthereby to produce individual water streams of different areas, thepressure drop across the plate and the turbulence introduced into thewater as it flows th-rough the holes being such that gases dissolved inthe water come out of solution as bubbles which enter said saltretention passages in the flowing water, the water downstream of theplate as it enters said salt retention passages of the filter elementwith said bubbles entrained in it being at a lower pressure than thewater upstream of the plate and its flow being in the form of turbulentstreams.
 6. The plant according to claim 5, wherein the plate comprisesa circular disc and the holes are arranged in the form of a spiralgenerated about the center of the disc.
 7. The plant according to claim5, wherein the plate comprises a circular disc and the holes arearranged in a series of circles the centers of which are coincident withthe center of the disc.
 8. A reverse osmosis desalination plant,comprising: a casing; a filter element located within said casing andconsisting of spirally wound reverse osmosis membranes in the form of aplurality of leaves attached to and wound around a permeate pipe anddefining a plurality of salt retention passages and a plurality ofpermeate passages each of which passages is of spiral form; a pump forpumping water to be desalinated to said filter element; a plate locatedwithin said casing and in the water flow path between said pump and saidmembrane, the plate obstructing water flow and having therein aplurality of holes which are circular in cross section whereby a seriesof streams of water are created downstream of the plate which impinge onthe end of the wound membrane and flow into the salt retention passages,the plate introducing turbulence into the flowing water and causing apressure drop, the pressure drop across the plate and the turbulenceintroduced into the water by the plate being such that gases dissolvedin the water come out of solution as bubbles which enter said saltretention passages in the water flow, the water downstream of the plateas it enters said salt retention passages of the wound leaves with saidbubbles entrained in it being at a lower pressure than the waterupstream of the plate and its flow being in the form of turbulentstreams.